The invention relates to a sequence casting process for the continuous production of a high-purity cast metal strand from a metal melt, preferably a steel melt, the metal melt being fed in controlled fashion from a melt vessel to a tundish and being discharged in controlled fashion from this tundish into a continuous-casting mold, and the supply of metal melt into the tundish being interrupted during the change of melt vessel, whereas the supply of the metal melt into the continuous casting mold is continued.
A sequence casting process is to be understood as meaning a casting process in which a plurality of metal batches, which are supplied to the casting installation in a plurality of melt vessels, are continuously cast to form a single metal strand without interruption to the casting process. It is in this case necessary for the melt vessel, after it has been emptied, to be exchanged for a further, full melt vessel within the shortest possible time. There is inevitably an interruption to the inflow of melt into the tundish, and it is necessary for the residual quantity in the tundish to be such that a sufficient quantity of residual metal melt is held in the tundish to span the changeover time which is needed before metal melt can flow into the tundish again from the further melt vessel which has been moved into the casting position. To maintain the continuous casting process during the changeover time, it is customary for the casting rate of the casting installation to be reduced during the changeover time. The changeover time can be kept very short using a ladle turning tower.
The continuous-casting installation itself can be equipped with a permanent mold of any desired design, such as for example one or more oscillating plate or tube molds, with caterpillar molds, with molds comprising rotating belts or molds which are formed by rotating casting rollers with insulating side walls. The cross-sectional format of the metal strand that is to be cast can also be set as desired, but especially when producing thin metal strips with thicknesses of less than 6.0 mm and widths of over 800 mm, particularly high demands are imposed on the starting phase or restart phase of the casting process after a ladle change, since in particular on account of the relatively small melt pool and the practically invariable metallurgical length until the kissing point in a two-roller casting installation, as well as the rapid full solidification of a thin metal strand, it is not possible to significantly reduce the casting rate. Furthermore, it is necessary to take into account the fact that during resumption of the supply of melt into the tundish, increased bath movement occurs in the residual metal melt, which is already covered with covering agent, and on account of the increased formation of waves at the bath surface, increased amounts of covering agent are introduced into the metal bath. Furthermore, when the ladle slide is opened, filling sand is introduced into the tundish, which requires a certain time and a calmer metal bath before it can float to the surface of the bath. The invention relates in particular to the casting of a metal strip using a two-roller casting installation based on the vertical two-roller casting process.
During production of a high-purity cast metal strand using any desired continuous casting installation, the liquid metal is usually fed from a casting ladle via at least one tundish or transfer vessel to a cooled permanent mold, in which the metal melt solidification process to form a metal strand is at least initiated. The transfer of the metal melt from the casting ladle into the tundish and from the latter into the permanent mold predominantly takes place through immersion pipes or shrouds, which, during steady-state casting operation are immersed in the melt pool of the vessel in each case arranged downstream and thereby allow flow and transfer of the metal melt into the permanent mold to be as calm and uniform as possible. The metal melt which has accumulated in the casting ladle, the tundish and, if appropriate in the permanent mold is usually covered by a layer of slag which protects the metal bath surface from oxidation. The basic arrangement of the melt-holding vessels in a multi-strand continuous casting installation for steel is known, for example, from U.S. Pat. No. 5,887,647. The more intensive the metal bath movement in the individual melt vessels, the more slag particles are introduced into the metal bath from the slag layer covering the metal melt, and the more particles of the refractory material from the lining of the melt vessels are also fed to the metal bath as a result of erosion. At the same time, the separation of particles of foreign material out of the metal melt at the metal bath surface or into the slag layer is impeded by excessively intensive metal bath movement. In the case of large-format metal strands, such as strands with slab cross sections, time for separating off foreign substances at the bath surface also remains in the permanent mold. In the case of small-format strands, and in particular strips with a low thickness, the introduction of foreign particles into the permanent mold should be avoided as far as possible, since the extent to which foreign particles can be separated off tends to be much more restricted in the permanent mold.
It is generally known that the quality of the cast strand is reduced if considerable fluctuations in bath level occur, as are inevitable during the starting phase of the casting process during initial filling of the tundish or as occur during a ladle change in sequence casting, wherein the metal melt which is held in the tundish is usually employed to span the ladle changeover time, and therefore casting is carried out with a continuously decreasing bath level. The stability of the melt flow in the tundish is greatly impaired as a result, and the metal melt is subject to undesirable introduction of slag.